This invention relates to the interconnections of individual lines and central switching apparatus of communications systems and particularly to protective circuits and devices for dealing with excessive voltages and currents which may appear on such lines.
The necessity for providing protection for communications apparatus and lines, or for any other electrical circuitry, for that matter, against damage resulting from excessive voltages and currents has long been recognized in the art. Such excessive voltages and currents may appear on either or both of the tip and ring conductors of a telephone system, for example, as power crosses resulting from direct metallic contact with a power distribution line. Power crosses may also occur as the result of magnetic induction arising from power line ground faults. In either case, power cross faults may seriously affect the operation of a switching system by causing the registration of false call originations, relay contact damage, for example, and in extreme cases, by presenting a fire and personal injury hazard. One common means for guarding communications equipment against damage is the carbon block protector connected to each of the tip and ring conductors of an individual line which, upon breakdown, operates to short foreign potentials to ground before damage can occur. Such protectors, however, frequently fail to operate as a complete safeguard since foreign potentials which fall short of the breakdown voltage may still be present to cause damage. Heat coils and fuses are also commonly employed as protective devices; these have the disadvantage in specific applications of possibly failing to respond in sufficient time to prevent circuit or equipment damage. Furthermore, since both excessive voltages and excessive currents may be involved, two separate protectors are generally required.
It is thus one object of this invention to provide a new and novel electrical circuit protective arrangement which responds rapidly and positively to both excessive voltages and currents of predetermined magnitudes.
Another object of this invention is a novel communication line protective circuit which responds rapidly and positively to both excessive voltages and currents to control the continuity of the line.
A further object of this invention is to provide a new and novel circuit fusing device.